THE PHYSICAL BASIS OF FERRO MAGNETISM 



31 



or 



5 = yikdIiaC) = 5o 



In iron and similar materials free from any considerable strain the 

 value of C is determined by the crystal anisotropy and is about 

 10^ ergs/cm.^ 6 « 10^ °K, a « 10~* cm. and the thickness of the 

 boundary layer comes out to be about 1000 atom diameters. This 

 value, probably correct as to order of magnitude, indicates that the 

 volume of the domain proper is much larger than that of the boundary 

 or transition region. 



At present it is not clear why application of an indefinitely small 

 field will not cause continual progression of the 180° boundary in one 



(a) 



(b) 



(c) 



^ 



^ 



Q 



C_^ 



Fig. 18 — The magnetic moment, M, and the moment of momentum, /, of an 

 electron in its orbit about the nucleus. A change in one moment entails a change 

 in the other, the (gyro magnetic) ratio remaining constant. 



direction so that one domain will disappear completely. The reason 

 for the non-occurrence of this progression except under certain circum- 

 stances is probably connected with the existence of strain gradients. 



Gyromagnetic Effect 



In the discussion of the structure of ferromagnetic atoms, use was 

 made of the concept of electron spin. This section will review the 

 evidence for the existence of this spin, its experimental determination, 

 and its relation to magnetic phenomena. 



Theory. In principle, the ratio of the moment of momentum to 

 magnetic moment may be determined as illustrated in Fig. 18, An 



